Further elaboration of true vs apparent wind

Posted by Grant Headifen on February 7, 2009 under Crew, Skipper | Be the First to Comment

There has been a request for further elaboration of the wind vectors as to why the true wind is always behind the apparent wind. Previously we used this diagram and the confusion is that the true wind vector is in front of the apparent wind vector.

true versus apparent wind vectors

The following diagram explains the vectors better perhaps. The answer is to look at the direction the arrows not their relative position on the diagram.

wind vectors, true versus apparent

wind vectors, true versus apparent

Fig A and B are the same. But now in Fig B it is easier to imagine standing on the boat and holding your arm out pointing to the direction of the wind that you feel (apparent wind). Then taking your arm and pointing further aft. This is the direction of the true wind. The rule is consistent. No matter which sail point you are on, the true wind is always behind (coming from further aft) than the apparent wind. Again, think of the car example in the previous blog on this topic.

This morming as I am writing this blog an intersting second example came up. A friend just called to ask if I’d like to play tennis. However, it is quite windy outside today (good day for sailing). In the mucho talk that proceeded, he said that he hits the ball so hard that the ball would not deflect very much in the wind. If his flabby muscles really could hit the ball that hard, then he is correct  due to this true vs apparent wind discussion here. If the (true) wind is blowing from across the court, a faster moving ball feels less side wind and more wind from the front. A slower ball would feel more of the side wind and be affected more. As the ball travels, the true wind is coming  from the cross court direction where as the apparent wind (what the ball feels) is from the direction of my flabby friend.

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